1. Field of the Invention
This invention relates to a method and an apparatus for the production by the sol-gel process of microspherical ceramic particles such as particulate ceramic fuels for use in nuclear reactors.
2. Prior Art
Particulate ceramic fuels for use in nuclear reactors are commonly produced by the sol-gel process, in which a stock solution or sol containing uranium and other fuel elements is treated with a second fluid (gas or organic liquid) to form small droplets under the resulting surface tension and a gelling agent such as ammonia is allowed to act on the droplets to gel (solidify) them, which are then heat treated for conversion to ceramic particles. This sol-gel process may be implemented by external or internal gelation. In external gelation, the gelling agent is allowed to act on the small droplets from the outside; in internal gelation, hexamethylenetetramine (HMTA) is mixed preliminarily with the stock solution or sol and the small droplets formed by interaction with the second fluid are heated to decompose the contained HMTA thermally and the decomposition product, ammonia, is used as the gelling agent.
According to a common method of internal gelation, heated silicone oil or the like is used as the second fluid and the small droplets of the stock solution or sol which are formed in that second fluid are heated up by about 80K so that HMTA is decomposed thermally, followed by gelation of the small droplets (see J. Nucl. Mater., 148 (1987) 324-331). Another approach that can be taken is the Ledergerber method, in which the small droplets of the stock solution or sol as formed within air are allowed to fall in the electric field of microwaves that have been generated within a cavity resonator (hereunder simply referred to as "resonator") so that they are heated dielectrically to decompose HMTA while gelling the small droplets (see Trans. Am. Nucl. Soc., 40 (1982) 55-56). A problem with the Ledergerber process is that the frequency specified for engineering purposes (0.915 or 2.45 GHz) is insufficient to provide a temperature elevation of about 80K that is held necessary to gel the small droplets of the stock solution or sol and the intended heating can only be accomplished by applying frequencies about ten times the engineering level (8.2-12.4 GHz).